1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof, more particularly relates to a semiconductor device capable of improving low forward voltage and low leak current characteristics of a Schottky barrier diode, and a manufacturing method thereof.
2. Description of the Related Art
FIGS. 6A and 6B show a conventional Schottky barrier diode 110. FIG. 6A is a plan view, and FIG. 6B is a cross-sectional view along the line C-C in FIG. 6A.
A substrate 21 is obtained by laminating a N− type epitaxial layer 21b on a N+ type semiconductor substrate 21a. On the N− type epitaxial layer 21b which is exposed through an opening of an insulating film 60, a Schottky metal layer 25 is provided, which forms a Schottky junction with a surface thereof. This metal layer is, for example, Ti. Furthermore, an Al layer to be an anode electrode 28 by covering the entire metal layer 25 is provided thereon. In a periphery of the semiconductor substrate 21, a guard ring 27 having P+ type impurities diffused therein is provided to secure a breakdown voltage. A part of the guard ring 27 comes into contact with the Schottky metal layer 25. On a rear surface of the substrate 21, a cathode electrode 29 is provided. This technology is described for instance in Japanese Patent Application Publication No. Hei 6 (1994)-224410 (Page 2, FIG. 2).
Moreover, a structure of a Schottky Barrier diode 120 shown in FIGS. 7A and 7B is also known. FIG. 7A is a cross-sectional view, and FIG. 7B is a partially enlarged view of the cross section.
A substrate 31 is obtained by laminating a N− type epitaxial layer 31b on a N+ type semiconductor substrate 31a. In the N− type epitaxial layer 31b, a plurality of P+ type regions 33 are provided. On the N− type epitaxial layer 31b which is exposed through opening of an insulating film 60, a Schottky metal layer 35 is provided, which forms a Schottky junction with a surface thereof. This metal layer is, for example, Ti. Furthermore, an Al layer to be an anode electrode 38 by covering the entire metal layer 35 is provided thereon. In a periphery of the semiconductor substrate 31, a guard ring 37 having P+ type impurities diffused therein is provided to secure a breakdown voltage. A part of the guard ring 37 comes into contact with the Schottky metal layer 35. On a rear surface of the substrate 31, a cathode electrode 39 is provided.
When a reverse bias is applied to the Schottky barrier diode 120, as shown in FIG. 7B, a depletion layer 40 is spread into the N− type epitaxial layer 31b from the P+ type regions 33. A distance between the adjacent P+ type regions 33 is set to not more than a width at which the depletion layer is pinched off. Accordingly, even if a leak current occurs in a Schottky junction region when the reverse bias is applied, the depletion layer 40 shuts off the leak current. Specifically, without giving too much consideration to leak current characteristics, as characteristics of the Schottky metal layer 35, one with low forward voltage characteristics can be selected. This technology is described for instance in Japanese Patent Application Publication No. 2000-261004 (Pages 2 to 4, FIGS. 1 and 3).
A factor in determining characteristics of the Schottky barrier diode 110 having the structure shown in FIGS. 6A and 6B is a work function difference φ Bn between the N− type epitaxial layer 21b and the Schottky metal layer 25. However, since the work function difference φ Bn is a value unique to metal, the characteristics are almost determined by the Schottky metal layer 25 to be used.
Furthermore, in the case of considering a certain Schottky barrier diode, when φ Bn is increased, a forward voltage VF of the Schottky barrier diode is increased, and, on the contrary, a leak current IR at a reverse voltage is reduced. Specifically, the forward voltage VF characteristic and the leak current IR characteristic are in a trade-off relationship.
Even if the structure utilizing the pinch-off of the depletion layer 40 as shown in FIGS. 7A and 7B is theoretically possible, it is difficult, in reality, to completely shut off a current path only by use of the depletion layer 40. The depletion layer 40 is generated by voltage application. Particularly, in a Schottky barrier diode with a breakdown voltage of about 40V, for example, since the epitaxial layer 31b has a low resistivity, the depletion layer 40 is unlikely to be spread and may not be spread sufficiently as designed. For example, in the structure shown in FIGS. 7A and 7B, if there is even one region where the depletion layer 40 is not spread sufficiently enough to be pinched off, it is impossible to suppress the leak current. Moreover, when the forward bias is applied, the P+ type regions 33 are regions which are not operated as the Schottky barrier diode. Specifically, if the P+ type regions 33 are provided too closely to each other in order to complete the pinch-off, an area of the Schottky junction between the Schottky metal layer 35 and the N− type epitaxial layer 31b is reduced. Accordingly, there arises a problem that the forward voltage is deteriorated and switching time is increased.